The pressure boundary of a steam turbine casing is penetrated by a rotating turbine shaft in order to transmit power outside of the steam environment. Consequently, the shaft must be sealed at the points of penetration to prevent the escape of steam from the casing at locations where 'the casing pressure exceeds ambient press tire and to prevent leakage of air into the casing at locations where the casing pressure is below ambient.
Many sealing systems have been employed in the past for this purpose. For example, labyrinth-type seals have been employed about the shaft. Because the teeth of the meshing but non-contacting labyrinth seals inherently form leakage paths past the seal, labyrinth sealing systems require elaborate steam seal and vent piping subsystems normally including a steam seal regulator, a gland exhauster and a gland condenser. Typically, labyrinth seals provided at the high-pressure end of the turbine prevent high-pressure steam from escaping the turbine. At the opposite end of the turbine, labyrinth seals prevent the entry of air into the turbine low pressure or vacuum region. At turbine start-up, the labyrinth seal at the steam inlet end of the turbine functions as a vacuum seal in view of the vacuum which exists throughout the turbine. As well understood, the labyrinth seals are thus used to restrict the flow of steam and/or air along the shaft.
Even with the small clearances between the teeth of the labyrinth seals, it is necessary to control the pressure differential across the labyrinth at the high pressure end of the turbine with a stream seal regulator and gland exhauster. Thus, a seal header is conventionally maintained at a predetermined positive pressure and will either supply steam to the annulus between the seals or dump steam from the annulus, depending the pressure differential across the labyrinth seals defining the annulus. Similarly, steam under positive pressure must be supplied the shaft seals adjacent the flow outlet of the turbine, such that the labyrinth seals can effectively preclude entry of air into the turbine. Further, there is some difficulty in matching the steam's temperature supplied to the labyrinth seals to the needs of the metal at those locations.
From the foregoing, it will be appreciated that use of labyrinth seals alone for sealing turbine shafts requires extensive piping subsystems with attendant design and installation problems. The cost of such piping and its ancillary controls, valves, gland exhaustera and the like, as well as the maintenance required for that equipment indicates a need for improved shaft sealing for turbines.